1. Field of the Invention
The present invention relates to motor protection systems. More particularly to a motor protection system for protecting a motor used for electric power steering for example by a motor overheating prevention function. In addition, the present invention relates to overheating prevention systems. More particularly, to a system that estimates the temperature of a motor and a motor peripheral device, limits current according to estimated temperature and protects the motor and the motor peripheral device from overheating.
2. Description of Background Art
An electric power steering system is known that applies a turning assist force by an electric motor to a steering shaft so as to facilitate steering when the steering shaft is turned and a vehicle is steered.
In JP-A No. 2005-324796, a control unit is described that estimates the temperature of a coil of a motor and executes a motor temperature protection control based upon the estimated temperature so as to prevent the overheating of the electric motor of an electric power steering system.
Generally, when the temperature of a coil of a motor is estimated, a value of current flowing in the coil and a resistance value of the coil are utilized according to Joule's law. More specifically, when the current value is I, the resistance value is R and energizing time is t, a calorific value Q can be estimated by an Expression 1 (Q=I×I×R×t).
The calorific value is estimated by this expression 1, however, to further estimate the temperature, the quantity of heat radiation is also required to be considered. The following expression 2 is an expression for estimating a calorie including a constant “a” as a term for correcting the quantity of heat radiation. A cumulative value T represents the temperature.
Cumulative value T=Σ(K×I×I−a)—(Expression 2). This Expression 2 is an expression for accumulating a calorific value when power steering is operated and electricity is supplied to the electric motor by energizing time and estimating the temperature and the constant “a” is subtracted as the quantity of heat radiation. The constant “a” in the Expression 2 is set to an extremely small value so that a cumulative value is zeroed in a longer period of time than the period of time until the temperature is restored to ordinary temperature from maximum temperature of the coil when the supply of electricity is stopped so as to estimate the temperature slightly higher and to secure temperature protection. The reason is that when the constant “a” is too large, the cumulative value T becomes small and the temperature of the coil is apt to be estimated lower. When no electricity is supplied for a long period of time, the cumulative value T is returned to zero because of the constant “a.” In the Expression 2, a coefficient K is an accumulating coefficient and is a numeric value acquired by an experiment beforehand so that a calculated value approximates to a measured value.
The temperature of the electric motor can be estimated using the Expression 2 without using a temperature sensor and the electric motor is protected by stopping the supply of current to the electric motor when estimated temperature is equal to or exceeds preset temperature.
The above-mentioned Expression 2 is suitable for a vehicle that dedicatedly runs on a maintained general road, however, it is not necessarily suitable for an all terrain vehicle (ATV) that runs off road and on other terrain. In off-road driving, as a steering angle is large, steering is frequent and the energization of the electric motor is frequent, the cumulative value T is excessive, the supply of current to the electric motor is stopped at low temperature at which protection from generated heat is actually not required, and no assist force may be applied to the steering shaft.
The reason why the cumulative value T is excessive and is not mutually related to the actual temperature is as follows. The constant “a” for correction in consideration of the quantity of heat radiation is an extremely small fixed value, however, the actual quantity of heat radiation varies depending upon difference between the temperature of the electric motor and ambient temperature. As the difference in temperature increases in operation over a long period of time, the quantity of heat radiation increases and the actual temperature of the electric motor hardly rises relatively. Therefore, according to the Expression 2 in which the constant “a” is the extremely small fixed value, a cumulative value of calorific values has a tendency to increase and it is conceivable that the correlation between the cumulative value T and the actual temperature cannot be maintained.
However, as the electric power steering system includes heating components by energization containing the motor (the coil and a brush of the motor) and a peripheral device such as a motor controller that controls the motor (more particularly, an electronic component such as FET), all these are objects for overheating protection. When plural elements or components (hereinafter merely called components) are all objects for overheating protection, heat gain/loss characteristics for every component are required to be considered. As heat capacity is different for every component, a certain component soon generates heat and soon radiates heat, another component slowly generates heat, and slowly radiates heat.
Then, when the heat gain/loss characteristics depending upon the difference in heat capacity are considered, estimated temperature is required to be determined so that the estimated temperature is not lower than the actual temperature of all components, that is, so that the estimated temperature rises sooner than the rise of the temperature of all components in generating heat and goes down more slowly than the decline of the temperature of all components in radiating heat.
However, when the heat capacity of each component is different, it is difficult to suitably estimate the temperature, for example, a component is estimated to be slightly overheated and a protection measure such as the limitation of current is sometimes taken soon. Then, though the temperature of the component actually goes down, a protective device is released and it is sometimes delayed to be restored to normal operation. In addition, as not only heat capacity but heat resistant temperature are different for every component, the temperature of plural components cannot be easily estimated by only one arithmetic expression.